Matching identifiers except set of keywords

[garretfick]

I'm trying to use rust-peg to parse IEC 61121 Structured Text (ST). You probably haven't heard of it, that's ok. I'm stuck trying to differentiate between identifiers and keywords.

For example, I have the rule to match an identifier, which is:

rule identifier() -> &'input str = $(['a'..='z' | '0'..='9' | 'A'..='Z' | '_']+)

However, there is a set of keywords that are not allowed as identifiers, for example while.

Presently, the grammar is matching while as an identifier. Is there some way to exclude a set of keywords (case insensitive of course) from a pattern match?

[garretfick]

I found an example that implements the desired behavior (excluding the case sensitivity part):

      rule keyword() = "while" / "other" / "keywords"

      rule identifier() -> &'input str = !keyword() i:$(['a'..='z' | '0'..='9' | 'A'..='Z' | '_']+) { i }